With the ubiquitous use of mobile communication devices such as smartphones and a rapid growth in mobile online communication applications, the leakage of privacy information has become one of the main issues to resolve in mobile communications. For example, when voice of the other party leaks out of a receiver speaker of a mobile phone during phone conversion, another person standing around or walking by the phone may hear or listen to the leaked voice, which may cause a privacy leakage issue. Also, the voice or sound leaking out of a receiver speaker of a mobile phone can sound as annoying noise to the other people who happen to be near the phone. For the purpose of avoiding the above issues, a user often manually tries to prevent the leakage of voice or privacy information, for example, by putting his/her hand over the phone or speaking in a lower tone. However, the other party's voice leaking out of the receiver speaker may not be properly controlled.